In general, a seat belt device is disposed in seats of various transportation vehicles to protect a passenger in an emergency such as a collision. Often, a buckle device is also disposed therein to facilitate carrying out an attaching operation or a detaching operation of the seat belt device. A typical buckle device is configured such that a spring urges a latch member for latching a tongue plate in a direction in which the tongue plate is latched and a lock member allows the latch member to maintain a latch state in which the tongue plate is latched to a buckle main body.
In such a buckle device, it may be necessary to display or otherwise indicate the latch state of the tongue plate with the buckle main body by using a lamp or to control an operation of a belt winding force via a retractor. Alternatively, it may be necessary to transmit information, regarding whether the seat belt device is attached or detached to an airbag ECU (electronic control unit). For this reason, a buckle device for detecting a latch state may be disposed inside the buckle main body (for instance, see Patent Documents 1 to 4 which are further identified below).
In the buckle devices disclosed in Patent Documents 1 to 4, a non-contact switch, in which a Hall element and a magnet are disposed adjacent to the buckle main body, is disclosed.
Specifically, the buckle device disclosed in Patent Document 1 is in an OFF state when the magnet disposed in an ejector is separated from the Hall element in a non-latch state. Moreover, the buckle device is in an ON state when the magnet is adjacently located just below the Hall element in a latch state. Since the Hall element is surrounded by a magnetic shielding frame, magnetism of an external magnetic field is shielded and the Hall element is restricted from being affected by an external magnetic field.
Additionally, the buckle devices disclosed in Patent Documents 2 to 4 provide a pair of magnets. When a latch member moves between a non-latch state and a latch state, the magnet opposed to the Hall element changes from the magnet on one side to that on the other side. Accordingly, a direction in which a magnetic field is applied to the Hall element changes, so that a direction of current generated by electromotive power of the Hall element reverses, thereby detecting whether the tongue plate is attached or detached. Patent documents 1 to 4 are respectively JP-A-2004-49358, JP-A-2004-135779, JP-A-2004-121602, and JP-A-2003-81057.
In the buckle device disclosed in Patent Document 1, although the Hall element is surrounded by the frame to restrict the influence of an external magnetic field, it is difficult to sufficiently restrict the influence of the external magnetic field. Additionally, in the buckle device disclosed in Patent Documents 1 to 4, since it is a proximity sensor for detecting whether the tongue plate is attached or detached on the basis of the movement of the magnet, magnetic flux density applied to the Hall element is dependent on a distance between the Hall element and the magnet. For this reason, it may be necessary to prevent an abnormal operation by setting a positional difference of the magnet to a small value.